This invention concerns acoustic flow meters. These meters are used for the measurement of flow velocity of fluids. They have particular application in meters for gas and water in pipelines. The invention also concerns a method of operating such flow meters.
A flow meter system consists of two acoustic transducers separated by a known distance within a tube through which the fluid flows. The measurement is of the time of flight of an acoustic wave packet between the two transducers in the direction of flow of the fluid and a second measurement of the time of flight in a direction opposite to the flow of the fluid. These will differ because the effective propagation velocity of the acoustic energy is the vector sum of the fluid velocity and the velocity of the acoustic energy in the fluid when it is at rest. The two unknowns of average acoustic velocity and average fluid flow velocity can be derived from these two measurements. In many practical situations sufficient accuracy is obtained by assuming that the fluid flow rate is directly proportional to the simple difference between the upstream and downstream times of flight.
Many variations exist in the physical arrangements of the two transducers and tube, and in the processing of the acoustic signals. Many of these variations are techniques to overcome problems which derive from multiple acoustic modes propagating in the tube, from multiple reflections within the tube, and from variations in flow velocity within the cross sectional area of the tube.
The core measurement in an acoustic flow meter is of the delay between transmission of the wave packet from one transducer and its reception at a second after propagation through the section of flow tube. The desired data are obtained from the time of arrival of a wave packet which consists of a number of cycles of oscillation whose amplitude commences at zero, rises to a maximum and then decays again to zero.
In the case of existing acoustic flow meters with multiple acoustic propagation modes and multiple reflections, only the first few cycles of the received wave packet are free from these effects and must be relied upon for the measurement of arrival time. In practice this leads to reliance on identifying just one zero crossing of the signal in the early part of the wave packet to estimate the instant of time of commencement of the received packet. To achieve this it is necessary to use high power driving circuits in the transmitters to create a rapid rise or onset in the received acoustic wave packet, and expensive and less sensitive wide band transducers in the receivers.
The invention, as currently envisaged, is an acoustic flow meter, including a tube fitted with three or more acoustic transducers distributed along the length of the tube and arranged to transmit wave packets up and down the tube between at least two pairs of the acoustic transducers, the wave packets comprising a signal including zero crossings spaced apart by a packet period determined by a known packet frequency; the flow meter also including measurement means associated with the transducers to make first measurements of received packets, from which the time of arrival of the zero crossings of a received packet can be determined relative to the time of transmission of the packet, and a second measurement from which a particular zero crossing in the transmitted packet can be identified in the received packet: and calculation means to determine the velocity of fluid flow in the tube using the measurements.
Measuring phase introduces ambiguity. The measured phase repeats at integer intervals of the period of the oscillation in the received waveform. The invention helps to resolve this ambiguity in the phase measurement and determine which zero crossing or cycle within the waveform is to be the reference time of arrival of the received wave packet.
By using narrow diameter tubes, lower frequencies, longer tubes, and careful design, it is possible to receive wave packets in which a significant number of cycles are free from any multiple reflection effects and are therefore available for analysis. Where only a single (zeroth) acoustic mode propagates it is possible to avoid contamination of the received wave packet by slower higher mode components. Under particular circumstances in the presence of higher order modes, the long tube can still provide cycles free from contamination by the slower higher order modes propagating in the tube.
A longer tube gives the freedom to design a tube in which effects of multiple reflections and higher order modes are minimised. As a result, a greatly increased number of cycles in the received packet become available for processing and much less reliance need be placed on the first one or two cycles and accurate control of amplitude. A consequence of these improvements is that cheaper yet more efficient, narrow band transducers can be utilised.
Distinct advantages arise using relatively small diameter tubing, relatively low acoustic frequency, and a relatively large distance between the two transducers. The small diameter increases the velocity for a given mass flow rate and this, along with the longer tube, increases the sensitivity of the instrument. There is also a minimisation of flow velocity variation across the cross-section of the tube. The increased pressure drop introduced by such a tube may or may not be important in specific applications.
An advantage of the invention is to enable accurate measurement of arrival time of wave packets reliant only on one, or more, zero crossings within the received waveform and not reliant on a rapid rise and known amplitude and shape of envelope.
Where there are a multiple of zero crossings which can be used as part of the phase measurement, this significantly increases the accuracy compared with use of just a single zero crossing.
The first measurements may be of the average phase of a series of zero crossings with reference to a clock signal of known periodicity.
The period of the clock of known periodicity may be the packet period.
The second measurement may be of zero crossings in packets having different packet frequencies.
The second measurement may be of phase difference between wave packets transmitted between two or more pairs of transmitting and receiving acoustic transducers.
The calculation means may rely upon the known distances, or ratio of distances, between the pairs of transmitting and receiving transducers.
The use of low acoustic frequencies and narrowband transducers has the added benefit of tending to reduce the power consumption within the electronics, which is an important consideration in battery driven meters designed for long field life.
The invention may therefore offer advantages of a reduction in signal processing power consumption, the ability to use cheap, narrow band and non-critical transducers, and the avoidance of need for accurate control of received wave packet amplitude and shape.
In a second aspect the invention is a method of operating an acoustic flow meter including a tube fitted with three or more acoustic transducers distributed along the length of the tube and arranged to transmit wave packets up and down the tube between at least two pairs of the acoustic transducers, the wave packets comprising a signal including zero crossings spaced apart by a packet period determined by a known packet frequency: the method comprising the steps of:
making first measurements of packets received at a transducer, from which the time of arrival of zero crossings of a received packet can be determined relative to the time of transmission of the packet;
making a second measurement from which a particular zero crossing in the transmitted packet can be identified in the received packet; and
calculating the velocity of fluid flow in the pipe using the measurements.